A photographic diffusion transfer process utilizing silver salts such as silver halides has heretofore been known. For such a photographic process, a method of directly obtaining a positive silver image on an image receiving element has been known in which a photosensitive element containing an exposed photographic silver halide emulsion and an image receiving element containing silver precipitation nuclei are overlaid on each other and an alkaline processing solution containing a silver halide solvent is coated between these two elements in the presence of a developing agent.
In this method, a positive image is formed directly when an unexposed silver halide emulsion in the photosensitive element is dissolved by the silver halide solvent and leached out as a silver ion complex into the alkaline processing solution, transferred to the image receiving element and precipitated in the image receiving element as a silver image due to the effect of the silver precipitation nuclei.
The layer constitution of the image receiving element comprises, starting from the support, an alkali neutralization layer containing an acidic polymer, a neutralization timing layer comprising cellulose acetate as the main ingredient and an image receiving layer containing silver precipitation nuclei as the fundamental elements, and the image receiving layer is prepared, for example, as described below.
Specifically, U.S. Pat. No. 3,179,517 describes a method of preparing a layer of regenerated cellulose by saponifying an acetyl cellulose film with an alkali, then immersing the regenerated cellulose layer into a gold salt solution and a reducing agent solution, thereby allowing a reaction to occur in the layer to form silver precipitation nuclei of colloidal gold. Further, Japanese Patent Publication No. 32754/69 describes a method of incorporating silver precipitation nuclei in an alkali impermeable polymeric substance by vacuum deposition, then solubilizing the silver precipitation nuclei in a solvent capable of dissolving the polymeric substance, coating the solution on a support followed by drying and then saponifying the surface of the polymer layer, thereby rendering the layer alkali permeable.
Further, Japanese Patent Publication No. 43944/71 describes a method of preparing an image receiving layer by forming silver precipitation nuclei in an acetyl cellulose solution, coating the nucleus solution on a support and then saponifying the acetyl cellulose into regenerated cellulose.
However, the silver iamge formed in the thus obtained image receiving element involves a drawback in that the image is liable to be discolored or faded or a drawback in that stains tend to be caused in white areas during preservation.
For improving such drawbacks, a method of coating a water-soluble polymer solution containing an alkali neutralization ingredient onto the surface of a silver image is described in Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789 and British Patent No. 1,164,642. However, since the method takes a considerable period of time until the surface coated with the aqueous polymer solution is completely dried and the surface is sticky and adhesive until the drying, printed matter cannot be laid over each other and finger prints or dust is often deposited. In addition, it is troublesome to coat such a solution on the silver image.
Further, with respect to the drawback that the transfer image is decomposed and turns yellow due to hydrogen sulfide present in the atmospheric air, although it has been disclosed in Japanese Patent Application (OPI) No. 10937/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") that the alkalinity in the image receiving layer is reduced, neutralized or rendered somewhat acidic, no sufficient effect can be obtained for various storage conditions by merely lowering the pH value.
Japanese Patent Publication No. 44418/81 discloses an image receiving element comprising, disposed on a support, (I) a layer of cellulose ester, polyvinyl ester or polyvinyl acetal which is hydrolyzable and becomes alkali-permeable upon hydrolysis containing a compound capable of diffusing and changing the property of the silver image, and (II) a regenerated cellulose layer containing silver precipitation nuclei formed therethrough. As compounds which are capable of diffusing and changing the property of the silver image, organic mercapto compounds have been described. In this case, a diffusion transfer processing solution and a mercapto compound in layer (I) are gradually diffused into layer (II) to protect the silver image formed in layer (II) and prevent discoloration or fading.
For completely attaining the effect of preventing discoloration or fading, it is necessary that the mercapto compound has a sufficient property of preventing discoloration or fading and that it remains in layer (I) during preservation of the undeveloped image receiving element and during the diffusion transfer process, penetrates from layer (I) to layer (II) after formation of the silver image by the diffusion transfer process to protect the image formed in layer (II). If the diffusion of the mercapto compound from layer (I) to layer (II) occurs before the completion of the diffusion transfer process, development is suppressed to entirely decrease the optical density of the transferred silver image on the image receiving element. Further, if the diffusion of the mercapto compound occurs too late, the image discolors or fades before the silver image is protected by the mercapto compound.
However, the mercapto compounds described in Japanese Patent Publication No. 44418/81 involve drawbacks in that they perform poorly for preventing the discoloration or fading, thereby causing image discoloration or fading and, in addition, the compounds diffuse from layer (I) to layer (II) while preserving the undeveloped image receiving element, thereby suppressing the development and reducing the optical density of the transferred silver image.
Further, Japanese Patent Application (OPI) No. 120634/74 describes an image receiving element prepared by using a homopolymer, copolymer and graft polymer of monoacrylate or monomethacrylate of a polyhydric alcohol as the polymer layer containing a compound capable of changing the property of the silver image.
However, the compounds described in Japanese Patent Application (OPI) No. 120634/74 also have a similar drawback to that of the compounds described in Japanese Patent Publication No. 44418/81 in that the preventing effect for the discoloration or fading of the image is insufficient, or the optical density of the transferred silver image is decreased while the undeveloped image receiving element is stored.
Further, use of a 2-mercapto-1,3,4-triazole derivative for obtaining a stable silver image in the diffusion transfer process has been disclosed in British Patent No. 1,276,961. Also, U.S. Pat. No. 3,655,380 discloses that 5-seleno-1,2,3,4-tetrazole derivatives can improve the tone of the silver image obtained by the diffusion transfer process into a medium gray tone and can provide a stable silver image as well.
Furthermore, it is also disclosed in Japanese Patent Publication No. 21140/81 and Japanese Patent Application (OPI) No. 500431/81 that the discoloration of the silver image can be prevented by acting on the silver image with a noble metal compound.
The above-mentioned organic mercapto compounds, however, involve a drawback in that the stabilizing effect for the silver images obtained by the diffusion transfer process is insufficient thereby causing discoloration or fading of the image. Further, it has also been found that these noble metal compounds have to be incorporated in a large amount in order to protect the silver image thereby causing stains or asserting undesirable effects on the image forming speed or velocity.
For preventing undesirable diffusion of the silver image stabilizers, Japanese Patent Application (OPI) Nos. 41041/85, 43658/85, etc., disclose attempts at disposing a hydrophilic polymer layer between an alkali impermeable polymer, controlling the saponification degree by utilizing the difference in the diffusion rate of the saponidying solution and, as a result, controlling the diffusion of the added silver image stabilizer. This method, however, brings about a problem in that the manufacturing process is increased by one step and the bonding strength between the hydrophilic layer and hydrophobic layer is weak thereby causing defoliation during production.
Further, an example of adding an image stabilizer to a neutralization layer or neutralization timing layer without disposing the above-mentioned hydrophilic layer is described in Japanese Patent Application (OPI) No. 231537/84 and the result of adding the image stabilizer to the neutralization timing layer and conducting a forcible degradation test for the transfer image is described in Example 4 of that patent. Although this can provide a remarkable effect for the forcible degradation test, the density just after the developing treatment is lower as compared with a sample containing a hydropholic layer as disclosed in Example 5 of the patent. This is due to the addition of water as the saponifying solution which increases the saponifying amount, and the problem cannot be solved by the mere distribution of the image stabilizer to the neutralization layer and/or neutralization timing layer in the case where no hydrophilic layer is present.
However, in the case of obtaining the regenerated cellulose by the method described above, the alkaline solution may sometimes penetrate as far as the neutralization timing layer lying below to partially saponify the cellulose acetate constituting the layer. It will be sufficient that only the image forming layer is saponified into a cellulose membrane. However, if the underlying neutralization timing layer is saponified as well due to the reason described above, the photographic properties, etc., of the image receiving element are adversely affected resulting in undesirable effects. For example, if the hydrophilic cellulose membrane is increased, since the alkaline processing solution coated between the image receiving element and the photosensitive element is adsorbed more on the side of the image receiving element, the image formation is retarded, or the storability after image formation is deteriorated. That is, discoloration and fading of the image when it is left in a light or a dark place is increased. Further, since the drying of the image forming layer just after the peeling of the photosensitive element and image receiving element is retarded, the surface of the layer is liable to be damaged and, depending on the case, the image is partially eliminated.
In order to remove such drawbacks, it is preferred that the saponifying reaction does not progress deeply and that the progress of the saponification from the image forming layer to the underlying layer is suppressed as much as possible.
As one of the means for suppressing the progress of the saponifying reaction from the image forming layer to the underlying layer, an alkaline solution-impermeable intermediate layer may be disposed between the image forming layer and the underlying layer (neutralization timing layer), so that the alkali solution does not permeate to the underlying layer. For example, in a case where the alkaline solution consists of a methanol solvent, the foregoing objects can be attained by using a hydrophilic polymer such as gum arabic, polyacrylamide and gelatin as the intermediate layer. However, this eventually increases the number of coating operations resulting in an economical disadvantage and a decrease in the performance due to the increase in the number of layers (for example, defoliation upon moistening).
As has been described above, it has been impossible in the prior art to provide an image receiving element with a sufficiently high maximum density of a gray (medium gray) tone maintained upon development and capable of forming a finished printed image which is stable for a long period of time upon storage, causes neither discoloration nor fading, results in no stains in the white areas and which can be obtained at a reduced cost.